Provided you are not worried about playback, the following advice might come in handy.
The Expression Tool lets you create your own expressions. Click the Expression Tool to turn it on. Double-click where you want to place your clef change. Select the Miscellaneous category and press the button labeled |Create Misc. Expression| This will bring up a window called Text Expression Designer
Set the Font to Maestro. Within the edit box, SHIFT-7 and SHIFT-? should help.
Mid-measure clef changes
Moderators: Peter Thomsen, miker